In the field of gas turbines for aircraft engines there has long been awareness of the need to increase performance by reducing weight as much as possible. In time, this resulted in the construction of stators that, on the one hand, are increasingly subjected to stress, namely high fluid-dynamic loads on the blades and, on the other, have increasingly smaller thickness profiles and therefore inevitably have low rigidity, both flexural and torsional.
The reduced rigidity of the stator blade arrays has, inevitably, resulted in the construction of turbines that have been found to be unstable under certain functional conditions.
It has been possible to experimentally establish that this instability was due to a marked sensitivity to aeroelastic phenomena deriving from reciprocal aerodynamic interactions between the blades of a stage with the consequent triggering of movements or vibrational states, known as “flutter”. This flutter stresses the blades, leading them into structurally critical conditions, as well as generating noise emissions.
Owing to these problems, the stator blade arrays were locally reinforced or made more rigid, but at the expense of an inevitable increase in weight and, in any case, the acceptance of geometries/configurations compromised in terms of aerodynamic performance and, consequently, of efficiency.
Patent application EP-A-2075413 teaches to optimize the aerofoils of the stator array. In particular, this array comprises aerofoils that can be different from one another. However, the solution presented in EP-A-2075413 is not very satisfactory with regard to parts assembly, as these parts are numerous and are different from one another. In other words, being made individually with different geometrical characteristics, these parts require dedicated storage and handling systems and different identification codes. In addition, in the assembly step, it is advisable to provide opportune reference and positioning systems to mount the various aerofoils in the correct position, as set by design.
Furthermore, in EP-A-2075413, the particular type of alternation of the aerofoils along the circumferential direction is not optimal from the standpoint of flutter stabilization, as it generates significant asymmetry in the blade array.